This invention relates to a gasket structure and the use thereof in a door structure for electromagnetically shielded enclosures, particularly radio frequency (RF) shielded enclosures.
A problem in the use of electromagnetic or radio shielded enclosures has involved the loss of shielding effectiveness due to the thin space between the door providing access to the enclosure and the door frame.
One approach to solving this problem involves the use of brass finger stock (thin brass with a row of curved fingers at one edge which make contact with the door and frame surfaces when the door is closed). Often an extended door edge in the form of a "knife edge" is used to mate with two strips of finger stock in a channel around the periphery of the door. When the door is closed, the "knife edge" is forced between opposing strips of finger stock in the channel which is mounted to the door stops. The only possibility for leakage of electromagnetic energy through the door is between the knife edge and the finger stock. When the finger stock has adequate spring tension and all mating surfaces are properly cleaned, this technique of shielding is adequate for many applications. After repeated usage, however, the finger stock tends to loose spring tension and surfaces become worn, corroded, and dirty with often a severe reduction in shielding effectiveness of the door. Therefore, maintenance is necessary to clean or replace the finger stock and/or knife edge to assure the desired level of shielding.
Another type of door which has seen considerable usage in recent years is an air expandable door which eliminates many of the problems of the door using brass finger stock. This door uses two panels at opposite surfaces of the door in an assembly which slides on rollers within a door frame. When the door is closed, air inflatable bladders within the door assembly are used to move the door panels away from each other and into contact with the door frame. Shielding is dependent upon continuous contact along the entire mating surfaces between the door panels and frame. In order to assure uniform contact, at least thirty PSI pressure is required on the door panels. Each panel mates with a door frame member around its entire periphery with the frame member width being about four inches. The total contact area is nearly 300 square inches for each door panel. The total force on the door frame is, therefore, approximately 18,000 pounds. Due to this high force level, the entire door and frame assembly must be constructed of very sturdy material, and is therefore quite heavy and expensive.
A third commonly used approach to providing sealing around doors for shielded enclosures involves the use of specially prepared gaskets between the door and the door frame. A mechanical means is used to provide pressure on the gasket so that the electrical conductivity through door surface, gasket and frame surface is as high as possible. A type of gasket often used is a tubular mesh gasket which is flattened between the door and frame. A disadvantage of this approach is that continued usage results in loss of the resiliency of the gasket with permanent deformation in a flattened state and reduction of conductivity when the door is closed. In addition, the pressure required is high so that the entire assembly must be sturdily constructed in order to assure adequate pressure.
Other structures have made use of inflatable members to press contacting members into engagement with door frames. The U.S. Pat. to Clark No. 3,507,974 uses inflatable tubing with flexible finger stock but use of the delicate finger stock here involves the same disadvantages discussed above with relation to the use of finger stock alone, i.e. loss of spring tension (which is needed here to disengage the door frame) and surface wear, corrosion and dirt accumulation.
The U.S. Pat. to Dunn No. 2,757,225 shows the use of inflatable tubing with thin flexible sheet metal as the contact member pressed so as to engage opposing metal sheeted surfaces.